Enhanced control of shuttle mass motion in mems devices

ABSTRACT

A MEMS device and a method of forming the same. A disclosed method includes: providing a silicon substrate layer, a buried oxide layer and a device silicon layer; using a microfabrication process to pattern a set of device features on the device silicon layer including a shuttle mass and an anchor frame; removing the silicon substrate layer and buried oxide below the shuttle mass; placing a shadow mask on a surface of the device silicon layer, wherein the shadow mask has a microscale opening to expose at least one device feature; and forming a nanoscale stopper on a sidewall of the at least one device feature by depositing a deposition material through the opening in a controlled manner.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional application, and claims the benefit, ofU.S. patent application Ser. No. 15/562,726, filed Sep. 28, 2017, whichclaims the benefit of International Patent Application No.PCT/US2016/026353, filed Apr. 7, 2016, U.S. Provisional PatentApplication No. 62/222,274, filed Sep. 23, 2015, and U.S. ProvisionalPatent Application No. 62/144,977, filed Apr. 9, 2015. The contents ofthese priority applications are incorporated by reference as ifdisclosed herein in their entireties.

FIELD

The subject matter of this invention relates to MEMS devices, and moreparticularly to MEMS devices having enhanced control of the shuttle massmotion using integrated displacement limiters with nanoscale resolutionand/or soft stoppers to provide frequency-up conversion.

BACKGROUND

Microelectromechanical systems (“MEMS”) are miniaturized devices, suchas sensors, valves, gears, mirrors, actuators, etc., embedded insemiconductor chips and typically integrated with electronic circuits.MEMS devices usually have a moving feature or component, e.g., acantilever, shuttle, diaphragm, etc., that moves in response to astimulus. Although they are designed to move in a restricted range,unpredictable external disturbances may cause the parts to exceed therange of motion they have been designed for, leading to device failure.Thus, there is a need to limit the displacement of microscale parts indynamic MEMS devices, which is typically accomplished using integrateddisplacement limiters, referred to as stoppers.

Stoppers are constructed on a device so that when the moving component(i.e., shuttle mass) reaches a predetermined displacement, physicalcontact is made with the stopper. The stopper is connected to a fixed ornon-moving portion of the MEMS device (e.g., an anchor frame) andabsorbs the impact of the shuttle mass, stopping it from moving anyfarther in the direction it was moving. The stopper defines both themaximum displacement of the shuttle mass and the minimum gap between theshuttle mass and anchor frame.

A widely used configuration for MEMS systems employs an interdigitatedcapacitor, which may, for example, be utilized to generate power inresponse to external forces, such as vibration or movement, or to sensea variety of external stimuli, such as pressure or acceleration. In mostcases, fabrication of these devices involves deep reactive ion etching(“DRIE”). In this process, silicon is etched through the depth of asilicon wafer with vertical sidewalls. Common features patterned in thisprocess include shuttle masses, spring beams, interdigitated electrodes,and stoppers.

A cross-sectional view of a typical fabrication process flow for asimplified MEMS device is shown in FIGS. 1A-1C. The illustrative deviceis fabricated from a silicon on insulator (“SOI”) wafer, encompassing asubstrate of silicon 14, a layer of buried oxide 12, and a top layer ofdevice silicon 10. In FIG. 1A, the device layer features, including theshuttle mass 16, anchor frames 18, spring beams and electrodes (notshown), etc., are patterned with DRIE as shown. In FIG. 1B, the siliconsubstrate 14 is patterned, for example removing the area under theshuttle mass 16 with DRIE. In FIG. 1C, the buried oxide layer isreleased with, e.g., hydrofluoric acid, resulting in a shuttle mass 16that can move between the two anchor frames 18.

In the process flow described in FIGS. 1A-1C, which is the typicalapproach used in fabrication of these devices, any additional desiredfeatures that might be utilized to improve performance, such asstoppers, are also defined in the lithography process and constructed byDRIB etching. Accordingly, such features are limited to the resolutionof the lithography technology, which is typically about 1 micron (μm) orlarger, referred to herein as microfabrication. Accordingly, currentMEMS fabrication techniques limit the size of such features, and thuslimit performance.

SUMMARY

Described herein are solutions for improving MEMS performance byenhancing control of the shuttle mass movement. One technique allows forthe fabrication of “nanoscale stoppers” and other MEMS features withnanoscale resolution using existing microfabrication techniques. Afurther technique includes the use of a structure referred to as a “softstopper” that provides up-conversion of the frequency response of a MEMSdevice.

To form nanoscale stoppers, after a device has been etched (i.e.,without stoppers), a shadow mask is aligned with the wafer and materialis deposited on the device via microscale openings etched from theshadow mask. The areas where nanoscale stoppers are to be formed areexposed to the depositing material. In many deposition processes, suchas plasma-enhanced chemical vapor deposition (“PECVD”), thermalevaporation, and sputtering, the material will deposit on all exposedfeatures, including sidewall features. The thickness deposited on thesidewall can be controlled, through deposition parameters (time, flowrates, temperature, etc.) with high precision allowing for nanoscaleresolution of the side wall deposition defining the nanoscale stoppers.

Soft stoppers can be implemented independently or in conjunction withnanoscale stoppers to increase MEMS device lifespan and increaseperformance with a frequency-up conversion that increases the bandwidthof the device.

In a first aspect, the disclosure provides a method of forming a MEMSdevice, comprising: providing a substrate, an insulator on the substrateand a device silicon layer on the insulator; using a microfabricationprocess to pattern a set of device features on the device silicon layerincluding a shuttle mass and an anchor frame; removing the substrate andinsulator adjacent the shuttle mass; placing a shadow mask on a surfaceof the device silicon layer, wherein the shadow mask has a microscaleopening to expose at least one device feature of the device siliconlayer; and forming a nanoscale stopper on a sidewall of the at least onedevice feature by depositing a deposition material through themicroscale opening.

In a second aspect, the disclosure provides a MEMS device, comprising: amovable feature that moves relative to a fixed feature; a nanoscalestopper that engages and prevents the movable feature from touching thefixed feature as the movable features moves toward the fixed feature,wherein the nanoscale stopper has a thickness of less than 1000nanometers; and a soft stopper that engages the movable feature beforethe movable feature engages the nanoscale stopper, wherein the softstopper slows the movable feature relative to the fixed feature.

In a third aspect, the disclosure provides a wireless microsensor,comprising: a sensor for sensing an environmental condition; and a MEMSdevice for harvesting energy and powering the sensor, wherein the MEMSdevice comprises: a movable feature that moves relative to a fixedfeature; a nanoscale stopper that engages and prevents the movablefeature from touching the fixed feature as the movable features movestoward the fixed feature, wherein the nanoscale stopper has a thicknessof less than 1000 nanometers; and a soft stopper that engages themovable feature before the movable feature engages the nanoscalestopper, wherein the soft stopper slows the movable feature relative tothe fixed feature.

In a fourth aspect, the invention provides a MEMS device, comprising: amovable feature that moves relative to a fixed feature; a stopper thatlimits motion of the movable feature in a first direction; and a softstopper that includes at least one cantilever beam that engages andslows the movable feature in the first direction before the movablefeature reaches the stopper.

BRIEF DESCRIPTION OF DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings in which:

FIGS. 1A, 1B, and 1C depict cross-sectional views of a process forfabricating a MEMS device.

FIG. 2 depicts a top view of a fabricated MEMS device according toembodiments.

FIG. 3 depicts a cross-sectional view of the MEMS device of FIG. 1Chaving nanoscale stoppers fabricated from coated sidewalls according toembodiments.

FIG. 4 depicts a top view of a MEMS device having a shadow mask used forfabricating nanoscale stoppers according to embodiments.

FIG. 5 depicts a top view of a MEMS device having stoppers formed usingthe shadow mask of FIG. 4.

FIG. 6 depicts a top view of a MEMS device having a shadow mask used forfabricating nanoscale stoppers on electrodes according to embodiments.

FIG. 7 depicts a graph showing normalized capacitance of nanoscalestoppers versus typical stoppers.

FIG. 8 depicts a top view of a MEMS device having a soft stopperaccording to embodiments.

FIG. 9 depicts a plot of output voltage versus frequency of a MEMSdevice.

FIG. 10 depicts a microsensor network having a MEMS device according toembodiments.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention, and therefore should not be considered aslimiting the scope of the invention. In the drawings, like numberingrepresents like elements.

DETAILED DESCRIPTION

FIG. 2 depicts an example of a power harvesting MEMS device 20, shown intop view, fabricated according to a microfabrication process such asthat described above. MEMS device 20 generally includes a movablefeature such as a shuttle mass 30, spring beams 22, interdigitatedelectrodes including fixed electrodes 26 and movable electrodes 28, andstoppers 24. The shuttle mass 30 is suspended by the spring beams 22,and the interdigitated electrodes 26, 28 form a capacitor. In-planemotion of the shuttle 30 will cause the gaps between the electrodes 26,28 to vary, which in turn varies the capacitance between them. When usedin conjunction with appropriate integrated circuits, the capacitancevariation may be used to increase in electrical potential of a storagecapacitor or a battery. Alternatively, a voltage applied across theelectrodes 26, 28 will produce a defined displacement of the shuttlemass 30.

The total device capacitance is calculated by adding the capacitancesbetween all the electrode pairs. As is known, achieving smaller minimumgaps during operation leads to larger maximum capacitance, andconsequently larger capacitance variation, which improves performancewhen employing these devices in most applications. In many currentdevice designs, the stoppers 24 on the device 20 define the minimumpossible gap between the electrodes 26, 28 and prevent device failureresulting from the moving 28 and fixed electrodes 26 contacting eachother. Thus, the thickness of the stoppers 24 dictates the maximumcapacitance, which in turn has a direct effect on device performance. Asnoted, feature size of stoppers 24 or the like are generally limited bythe resolution of the technology, i.e., 1 μm or more using microfabrication techniques based on deep reactive ion etching (“DRIE”).

The present approach allows stoppers 24 and other such features to befabricated with nanoscale resolution (e.g., less than 1000 nanometers)using existing microfabrication techniques based on thin filmdeposition. FIG. 3 depicts an example of this process with reference tothe generic MEMS device depicted in FIG. 1C. In this new process, afterthe MEMS device has been etched (without stoppers, etc., as shown inFIG. 1C), a shadow mask 30 is aligned with the wafer and stoppermaterial is deposited on the device silicon 10, such as that shown inthe cross-sectional (side) view of FIG. 3. The shadow mask 30 hasmicroscale openings 34 that allow a deposition material 32 to go throughand deposit on the sidewalls 37 of the shuttle mass 16 and anchors 18.Using a technique where a controlled amount of deposition material 32 isprovided, nanoscale features, such as stoppers and the like can beformed on any of the sidewalls 37 of the device.

This process results in nanoscale stoppers 35 being formed on thesidewalls 37 between the anchor frames 18 and shuttle mass 16, which areexposed to the deposition material 32. In many deposition processes,such as plasma enhanced chemical vapor deposition (“PECVD”), thermalevaporation, and sputtering, the deposition material 32 will deposit onsidewall features, although at a slower rate compared to the topsurfaces. The thickness deposited on the sidewall can be controlled,through deposition parameters (time, flow rates, temperature, etc.) withhigh precision allowing for nanoscale resolution of the side wallthickness defining the nanoscale stoppers 35.

FIG. 4 shows an energy harvesting MEMS device similar to that depictedin FIG. 2 with a microscale shadow mask 60 on top. As can be seen, themask includes two openings 54 through which deposition material isallowed to pass. The size and location of the openings 54 may changedepending on the application and parameters used during the depositionprocess. For example, rather than locating stoppers at the ends of theshuttle mass, stoppers may be placed at other locations, such as on theelectrodes 26, 28 (FIG. 3).

FIG. 5 shows a resulting device with the sidewall deposited nanoscalestoppers 40A, B. In this example, nanoscale stoppers 40A, B limit motionin each direction and include a section formed on the shuttle mass 30and a section on each anchor 50A, B. Thus, for example, each stoppersection may be implemented with a 50 nanometer thick material 51 on eachsidewall, such that the total limit for the displacement of the shuttlemass in either direction is 100 nanometers. It is, however, understoodthat stoppers 40A, B may be implemented with different dimensions.

It is also understood that the stoppers 40A, B may be located elsewhereon the MEMS device. For example, FIG. 6 depicts a shadow mask 62 placedon top of the device silicon layer having openings 56 over theinterdigitated electrodes 26, 28. Using this configuration, stoppers canbe formed on the sidewalls of the interdigitated electrodes 26, 28. Withthis configuration, stopper material applied through the shadow mask 62will coat the sidewalls of the interdigitated electrodes 26, 28 with apredetermined thickness, thus allowing a coating on the electrodes 26,28 to act as stoppers.

Regardless of the location, the stopper material may comprise anymaterial deposited in a cleanroom environment that can coat sidewallsincluding, e.g., silicon oxide (“SiO₂”), silicon nitride (“SiN”), andparalyne. The stopper thickness is controllable and dependent on thedeposition parameters such as deposition time, gas flow rates, andtemperature. The shadow mask 62 may utilize any microscale geometry thatexposes sidewall features in a MEMS device. As noted, the nanoscalestoppers may be located anywhere on the device, including, e.g., theshuttle, electrodes, springs, etc. The use of nanoscale stoppers may beapplied to limit in-plane motion, as well as limit linear or angularmotion. The described approach thus allows for the control of minimumgap and maximum displacement with nanoscale resolution. Illustrativeuses of MEMS fabricated with this process include sensors, microphones,accelerometers, gyroscopes, actuators, power harvesters, seismic sensors(e.g., for oil and gas exploration), motes, personal devices, smartclothing, etc.

As noted, one advantage of nanoscale stoppers is the ability to controlthe minimum gap between interdigitated electrodes 26, 28. This isequivalent to controlling the maximum capacitance and can haveapplication in any MEMS device that uses variable capacitors such aspressure and force sensing, actuation, or power harvesting. A comparisonof the capacitance of a MEMS device versus shuttle mass position withtypical micro-fabricated stoppers and the new nanoscale stoppers isshown in FIG. 7. The microscale stoppers allow for a minimum gap ofabout 1 μm and the nanoscale stoppers allow for a minimum gap of under100 nm and as little as 10 nm or less. The ability to reduce the gapgreatly increases the effectiveness of the device. For instance, a 100nm minimum gap results in a 10× increase in maximum capacitance relativeto a one micron gap, thus an order of magnitude increase in deviceoperational range.

Soft Stoppers

In a further embodiment, performance may also be enhanced by employing a“soft stopper” in a MEMS device, which slows down the shuttle massbefore the electrodes reach maximum displacement. The soft stoppersserve various functions. First, when they are implemented in conjunctionwith nanoscale stoppers, they will decrease the force before the impactof the shuttle mass (or electrodes, etc.) with the nanoscale stoppers,which will help decrease the wear that the nanoscale stoppers experiencedue to the impact and thus increase the lifespan of the device.Secondly, soft stoppers can be used to increase the operationalfrequency range of the device resulting an effect referred to asfrequency-up conversion of the device resonant response. The latter cansignificantly improve the performance of the device in manyapplications. For example, power output and performance of energyharvesting MEMS devices is directly proportional to device frequency:higher frequency results in higher power. Finally, soft stoppers canalso help prevent device failure due to pull in, which occurs when themoving electrodes get “stuck” in position near the stationaryelectrodes.

In one illustrative embodiment, soft stoppers can be achieved by etchingone or more cantilever stopper beams on the anchor frame that impact theshuttle mass before the displacement maximum is reached. On impact, thebeams deflect in a manner similar to the primary flexures or springbeams that support the shuttle mass, increasing the overall stiffness ofthe system.

FIG. 8 depicts an illustrative embodiment of a MEMS device havingcantilever stopper beams 70 attached to the anchor frame 72. Thecantilever stopper beams 70 may optionally include protrusions 75 andcorresponding indents at contact zones 76 in the shuttle mass 74.However, such protrusions and indents are not required.

The increase in stiffness serves several purposes as mentioned herein,including:

1. When soft stoppers are implemented in conjunction with nanoscalestoppers, they reduce the shuttle mass velocity before impact with thenanoscale stoppers, thus reducing impact forces which can cause wear andtear.

2. By increasing the stiffness of the device, the operational frequencyrange of the device is increased resulting in an effect referred to asfrequency-up conversion. This is because higher spring stiffnesscorrelates to higher resonant frequency. In general, if an increase indisplacement causes an increase in spring stiffness, this is calledspring hardening, a phenomenon thoroughly studied in dynamics andmechanical systems, which can result in frequency-up conversion,increasing the operational bandwidth of the device.

3. The soft stoppers can provide an opposing force against theelectrostatic pull-in force from the charged electrodes. If pull-in doesoccur, the springing effect of the soft stoppers can aid in pushing theshuttle mass 74 in the opposite direction.

In the example of FIG. 8, cantilever beams 70 are etched in the anchorframe and a contact zone 76 is optionally etched in the shuttle mass 74.Each beam 70 may include a protrusion at the end to initiate contact.The displacement at which the beam contacts the cantilever beam (e.g.,at least one micron) is less than the maximum displacement determined bythe nanoscale stoppers, e.g., less than 1000 nanometers (not shown). Thestiffness of the beams can be controlled by their geometry anddimensions, therefore allowing for flexibility in design choice.Although shown as a pair of beams, any number of beams may be utilized.

FIG. 9 shows the voltage output of a resonating MEMS device during avibration frequency sweep at different acceleration levels in mili-g(“mg”) (where g is the gravitation acceleration constant 9.81 m/s²). Thelong, slanted curves increase in amplitude as the frequency sweeps up.With an increase in the vibration acceleration level the bandwidth alsoincreases, allowing for a larger range of driving frequencies that cancause the device to resonate.

FIG. 10 depicts a network of wireless microsensors 80, 80 a, 80 b. Eachmicrosensor is self-powered with a power harvesting MEMS device 84 thatgenerates power in response to an external force, such as a vibration.Each MEM device 84 includes at least one of a nanoscale stopper and/or asoft stopper, as described herein. As shown, a sensor device 82 isprovided that senses one or more environmental conditions (e.g.,temperature, air flow, light, pressure, etc.) and communicatesinformation either to other wireless microsensors in the network ore.g., a control system 88. Such a network can be used in any number ofapplications, e.g., smart buildings, vehicles, smart appliances, objectpart of the Internet of Things, smart clothing, etc.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims. Note that for the purposes of thisdisclosure, the term shuttle mass refers to any type of movablecomponent in a MEMS device.

What is claimed is:
 1. A microelectronic mechanical system (“MEMS”)device, comprising: a movable feature that moves relative to a fixedfeature; a nanoscale stopper that engages and prevents the movablefeature from touching the fixed feature as the movable feature movestoward the fixed feature, wherein the nanoscale stopper has a thicknessof less than 1000 nanometers; and a soft stopper that engages themovable feature before the movable feature engages the nanoscalestopper, wherein the soft stopper slows the movable feature relative tothe fixed feature.
 2. The MEMS device of claim 1, wherein: the movablefeature comprises a plurality of movable electrodes extending from ashuttle mass; and the fixed feature comprises a plurality of fixedelectrodes interdigitated with the plurality of movable electrodes. 3.The MEMS device of claim 2, wherein the nanoscale stopper is positionedon sidewalls of the shuttle mass and an adjacent anchor frame.
 4. TheMEMS device of claim 2, wherein the nanoscale stopper is positioned onsidewalls of the plurality of movable electrodes and plurality of fixedelectrodes.
 5. The MEMS device of claim 1, wherein the soft stoppercomprises at least one cantilever beam positioned on an anchor frame. 6.The MEMS device of claim 5, wherein the at least one cantilever beamincludes an end region having a protrusion located to engage a contactzone on the shuttle mass.
 7. The MEMS device of claim 1, wherein thenanoscale stopper is fabricated from a material selected from a groupconsisting of: silicon oxide (“SiO₂”), silicon nitride (“SiN”), andparalyne.
 8. A wireless microsensor, comprising: a sensor for sensing anenvironmental condition; and a microelectronic mechanical system(“MEMS”) device for harvesting energy and powering the sensor, whereinthe MEMS device comprises: a movable feature that moves relative to afixed feature; a nanoscale stopper that engages and prevents the movablefeature from touching the fixed feature as the movable feature movestoward the fixed feature, wherein the nanoscale stopper has a thicknessof less than 1000 nanometers; and a soft stopper that engages themovable feature before the movable feature engages the nanoscalestopper, wherein the soft stopper slows the movable feature relative tothe fixed feature.
 9. The wireless microsensor of claim 8, wherein: themovable feature comprises a plurality of movable electrodes extendingfrom a shuttle mass; and the fixed feature comprises a plurality offixed electrodes interdigitated with the plurality of movableelectrodes.
 10. The wireless microsensor of claim 9, wherein thenanoscale stopper is positioned on sidewalls of the shuttle mass and anadjacent anchor frame.
 11. The wireless microsensor of claim 9, whereinthe nanoscale stopper is positioned on sidewalls of the movableelectrodes and fixed electrodes.
 12. The wireless microsensor of claim9, wherein the soft stopper comprises at least one cantilever beampositioned on an anchor frame.
 13. The wireless microsensor of claim 12,wherein the at least one cantilever beam includes an end region having aprotrusion located to engage a contact zone on the shuttle mass.